In this proposal we will attempt to synthesize, characterize, and explore the reactivity of porphyrinato organoimido complexes of ruthenium and osmium. These complexes may serve as models for intermediate species which form during metabolic cycles such as the reduction of nitrite to ammonia catalyzed by nitrite reductase, an important but not well understood enzyme in the plant kingdom. The study of these model metalloporphyrins may provide needed information for bio-inorganic chemistry. This proposal will proceed in three stages. The first stage will involve the synthesis of porphyrinato ruthenium and osmium organoimido complexes. Two major approaches will be attempted: the use of oxo-exchange reagents such as phosphinimines, isocyanates, and silylated amines and the use of reactive ruthenium/osmium porphyrinato precursors. The second stage will focus on the spectroscopic characterization of the products to determine whether they contain metal-N bridged nitrene fragments, one doubly bonded organoimido ligand, or two doubly bonded organoimido ligands. In the final stage the reactivity of the porphyrinato ruthenium and osmium organoimido complexes will be explored and related to possible intermediate species of the nitrite reductase cycle. Specifically, we will examine the fate of the organoimido fragment upon the addition of acid and/or reducing equivalents to determine if amines or coupling products such as azoarenes or azoalkanes predominate and we will explore the use of these organoimido complexes as potential nitrene transfer reagents for the aziridination of alkenes.